In duplex transmission of data over a two-wire line, i.e. when there is simultaneous transmission in both directions (upstream and downstream), both the useful signal originating from the opposite station and echoes resulting from the receiver's own transmitter occur at the respective receiver inputs.
Whereas the disruptive echoes can be suppressed by selective filters in the case of frequency division multiplex transmission methods, special devices for echo compensation are necessary in the case of common-frequency transmission methods.
According to the prior art, transmission systems with echo compensation have been embodied with a basic arrangement as illustrated here in FIG. 1. As the echoes which occur depend to a very great extent on the analog wiring and the parameters of the line used, it is generally necessary to perform adaptive adjustment of the echo compensator.
According to the prior art, this echo compensation is performed by simulating the echo pulse response by means of an FIR filter. Filters with a finite pulse response, that is to say nonrecursive filters, are referred to as FIR filters. The convolution of the transmission signal with the echo pulse response then yields the echo to be compensated. The FIR filter can be embodied such that it is either fixed or adaptively adjustable. Preferably the gradient algorithm is used for the adaptation owing to its ease of implementation. This algorithm is described, for example, in the teaching manual “Digital filters-Analysis, Design and Applications” by Andreas Antoniou (ISBN 0-07-002121-X) on pages 625 to 634. The length of the echo pulse response determines here the necessary number of coefficients of the FIR filter.
According to the prior art, when there are correspondingly long echo pulse responses, a very large number of coefficients of the FIR filter are produced. As a result, the adaptation of the filter is very slow and the filter consequently has very poor convergence characteristics.